1. Field of the Invention
The present invention relates to an optical network unit of an Ethernet passive optical network and a control method thereof. More particularly, the present invention relates to an optical network unit of an Ethernet Passive Optical Network (EPON) that monitors optical-output control signals in upstream transmission so as to prevent a malfunction thereof in advance. In addition, the present invention relates to an optical network unit of an EPON that prevents influences from being exerted on other optical network units operating normally when an abnormal operation occurs, and a control method of the optical network unit in an EPON.
2. Description of the Related Art
At present, Asymmetric Digital Subscriber Line (ADSL) and cable modem systems are the most widely used for high-speed Internet services. The ADSL system uses existing telephone lines, and provides high-speed Internet services with speeds between 2 Mbps and 10 Mbps through the ADSL modem installed in each subscriber's computer. Users typically install filters on the telephone line immediately prior to entry into a telephone, to filter the noise from ADSL traffic that can adversely affect operation telephones, faxes, etc.
On the other hand, cable modems use existing coaxial cables installed for cable TV services in order to provide high-speed Internet services, so a subscriber must install a cable modem within the area of his/her PC when already subscribing to cable TV service in order to be provided with the high-speed Internet services, or would have to purchase additional equipment, such as a wireless router that receives an output of the cable modem, and install a wireless network card into their pc. Otherwise, coaxial cables output from the modem would have to be run throughout one's house, which is unsightly and labor intensive.
These high-speed Internet services are satisfactory in performance in providing services such as Internet web surfing (HTTP), E-mail, file transfer (FTP), etc. with much higher transmission capacity of 2 to 10 Mbps, as compared with the existing telephone line modem having a speed of 56 Kbps, but they still have a limitation in meeting the users' emerging requirements such as VoIP (Voice over Internet Protocol), VoD (Video on Demand), Internet broadcasting service, etc.
Moreover, high-speed Internet service using the cable modem has a disadvantage in that the bandwidth which can be provided decreases as the number of subscribers increases, and the high-speed Internet service using the ADSL scheme has a disadvantage in that the bandwidth which can be provided decreases as the distance between a telephone office and a subscriber network increases. Also ADSL has problems associated with adverse weather conditions, for example, many subscribers to ADSL are aware that during a thunderstorm it is not unusual for the ADSL connection to be interrupted.
In an attempt to solve these problems, there have been proposed FTTH (Fiber To The Home), FTTB (Fiber To The Building), FTTC (Fiber To The Curb), etc., in which optical cables are installed to the subscriber's in-home network. In addition, studies are being conducted about an Ethernet passive optical network (E-PON) for the sake of enhancing the price-to-service ratio.
More particularly, the E-PON is an Ethernet-associated network which is constructed with passive elements without using power-consuming active elements in the optical subscriber network, so as to enhance its price competitiveness. The standard for the E-PON is being established by the IEEE (Institute of Electrical and Electronics Engineers) 802.3ah Ethernet in the first Mile Task Force. An example of the E-PON is illustrated in FIGS. 1A and 1B and discussed herein below.
FIGS. 1A and 1B are block diagrams illustrating respective flows of upstream and downstream traffic in a conventional E-PON. It can be seen from FIG. 1A that an E-PON has a point-to-multipoint structure in which a plurality of Optical Network Units (ONUs) 20-1 to 20-3, etc., are connected to one Optical Line Terminal (OLT) port 10 through a splitter 30, which is a passive element. Data transference between the OLT 10 and ONUs 20 is performed in units of an Ethernet frame. For example, downstream signals from the OLT 10 to the ONUs 20 are transmitted as broadcasting data, and upstream signals from the ONUs 20 to the OLT 10 share bandwidths allocated to the ONUs 20 by the Time Division Multiple Access (TDMA) scheme.
Therefore, still referring to FIGS. 1A and 1B, upon transmitting upstream signals to the OLT 10, when the conventional ONUs 20 are granted by the OLT 10 so as to collide between each other in a burst mode scheme, the ONUs 20 will transmit IDLE data (data for clock recovery time and code group arrangement in the OLT) to the OLT 10, and then transmit corresponding data as upstream frames in the TDMA scheme.
More particularly, each ONU 20-1, 20-2 and 20-3 transmits frames upstream to the OLT 10 in the TDMA scheme. Each ONU 20-1, 20-2 and 20-3 is allocated with a time period (time slot) from the OLT 10, and the individual ONU can transmit a corresponding frame only during the time period (time slot) allocated, as other time slots are allocated to other ONUs, etc. Upon an upstream transmission, when the on/off control of a laser diode is operating in a normal state, the individual ONUs do not transmit outside (beyond) their allocated time slot. However, when the on/off control of the laser is not properly performed due to a malfunction of a particular ONU (such as 20-2), abnormal data is transmitted in excess of a preset time period.
The extended transmission beyond the respect duration of the time slot by the malfunctioning ONU exerts an influence upon time periods allocated to the other ONUs 20-1 and 20-3 such that the other ONUs 20-1 and 20-3 recognize that the malfunctioning ONU 20-2 is continuously occupying the transmission line, thereby causing a serious error in data transmitted in the upstream direction.
While ONU #1 20-1 successfully transmits an ONU #1 data frame during a preset time, that is, during a first time period, ONU #2 20-2 cannot transmit ONU #2 data within a second time period due to an error occurring in ONU #2 and occupies the transmission line, even after the second time period has elapsed. That is, ONU #2 20-2 continuously transmits the erroneous ONU #2 data in excess of a preset time period, so that data collision occurs in a third time period during which the ONU #3 20-3 must be transmitting ONU #3 data, and such data collision continuously occurs, thereby causing the overall Gigabit E-PON to be unable to transmit data normally. If the malfunctioning ONU#2 20-2 continues to transmit beyond the allocated time slot, the data collisions can slow down or essentially impair the communication capability of the other ONUs that are not malfunctioning.